Soil is no doubt an essential component of an
agricultural system; therefore, the maintenance of soil quality by
proper management is important for successful crop growing. While soil
provides with the essential requirements like water and nutrients,
changes in the basic soil reactions through excessive cultivation and
degradation of the soil conditions for supporting crops do take place
constantly. Loss of soil quality therefore reduces crop production. The
challenge lies in the development of technological innovations to combat
the food inadequacy for the expanding population, for the decades ahead.
The science of food production demands adequate inputs, and at the same
time, the environment has to be protected through proper management.
Foliar application of nutrients nevertheless, is no substitute for soil
fertilization, and yet, it can be alternative where crop needs are
critical and soil application is expensive, time-consuming and many
times ineffective.

The leaves of plant are capable of absorbing
nutrients supplied in a liquid or aqueous medium. This capacity is
exploited in many agronomic practices like application of inorganic
nutrients, growth regulators and herbicides, for the purpose of
enhancing crop production. Foliar supply of nutrients have many
advantages over the root-feeding. There have been considerable interest
in the practical use of this technique as also several accomplishments
not only in the understanding of the mechanisms involved in foliar
uptake, but also in the development of chemical and surfactants or the
greater effectiveness.

Several inorganic nutrient elements are generally
required by plants for their growth and development. Presently, 25 per
cent of these have been identified. Plant roots are the organs which
have two important roles, for providing anchorage and for abstracting
water and nutrients from the soil. The root and shoot of the plants are
mutually dependent. Thus, the shoot receives the inorganic nutrients
from the root, the metabolites from the shoot are translocated to the
root. The medium for plants contains all the factors/necessary for
growth-water, carbon dioxide, inorganic salts and diffuse sunlight. All
the plant parts are able to carry on the two vital processes — the
absorption of solutes and the photosynthesis.

The evidence that the leaves could be feed with
inorganic salts effectively came from a number of scientists in the last
century. Reports on the beneficial effects of foliar feeding with
compounds containing N, Mn, Ca and B on several crops appeared during
the next few decades. Urea sprays for N were practice with many crops in
several countries. The introduction of radioisotopes in the early 1950s
marked the beginning of our understanding of foliar uptake of inorganic
nutrients and organic substances. The need to exploit the capacity of
plant leaves to absorb inorganic nutrients has increased greatly for a
number of reasons (i)
adverse soil conditions, which favour fixation of nutrients and thus
render many essential ones unavailable for root absorption (ii)
root absorption is slow for some elements and also results in poor
translocation (iii) relatively large amounts of fertilizers are required
for root supply and heavy application loads to soil-water pollution.
Foliar supply of nutrients can result in increasing the photosynthetic
efficiency and it is possible to modify the physiology of the leaf.

Out knowledge on the mechanism of foliar uptake
cuticular penetration, absorption by the leaf cells, cell-to-cell
transport within the leaf and the transport out of the leaf to other
regions has widened in the last few years. Furthermore, there has been a
revival interest in this field, both for basic research and the
practical application.

MECHANISM OF FOLIAR UPTAKE:

The inorganic nutrient elements given
in the form of aqueous sprays have to be first absorbed by the plant
leaf before translocation to other ports. The mechanism of absorption by
the leaf cells has been studied using leaf disks and leaf slices. Light
is found to enhance the absorption. The relative mobility of various
elements was studied as early as in 1957, with the aid of radioisotopes
and has been grouped into 3 categories based on their rate of transport
out of the leaf. Since then, the techniques of application and
measurement of transport have been refined by several workers and based
on these, the relative mobility of the elements can be grouped as
follows: i)
Freely mobile: N, P, K, Rb, Na, Mo; ii)
Partially mobile: Fe, Mn, Zn, Mo, Cu, B; iii)
Relatively mobile: Ca, Mg. There are environmental factors like light,
temperature and relative humidity, which directly and indirectly affect
foliar absorption. Young leaves have been found to absorb nutrients more
effectively than the one mature ones.

Of the major nutrients, N has been extensively used
for foliar sprays for many crops and urea has been the source. Urea
sprays for fruit trees, and crops like sugarcane and pineapple have been
in commercial practice in many countries for a long time. Workers
revealed that post harvest urea sprays resulted in greater N uptake and
distribution to the new growth in apples. The interest in foliar
application of N has been renewed in the last 3 decades, especially for
timely supply of N to soybean during the 'grain-filling' period. There
are two contrasting features in soybean with regard to N nutrition.
Being a leguminous crop, N application depresses N fixation. On the
other hand, the N requirement for soybean is one of the highest amongst
the field crops.

Foliar nutrition studies have been extensively
carried out and application of N sprays to cereals is a regular practice
particularly for increasing the longevity of the leaves. Study indicates
that foliar sprays with N, P, K and S at the late reproductive stage are
superior to early application on the ear length of corn. Foliar sprays
of urea during winter are effective in increasing N levels and not the
yield of passion-fruit. Foliar application of P and K has been reported
to benefit the groundnut crop grown under salt stress. A new role for
foliar nutrition is in the regulation of water use efficiency of fruit
trees. Sprays with complete nutrients and with KCl help apple trees
withstand water-stress. Studies have shown that foliar application of N,
P, K and S increase grain yield of corn.